A musical, in the end, is only as good as its book. For all the liveliness of Stephen Flaherty's score and the liberal intentions behind a show first seen in New York in 2005, Lynn Ahrens's libretto is too complicated for its own good. Ahrens has clearly tried to cram in too much of the novel by Sherley Anne Williams on which her story is based, with the result that the show, in its treatment of race relations, is more exhausting than spiritually enlightening.

Set in the antebellum south of 1847, the plot deals with the wildly different but ultimately interconnected lives of two women: Dessa Rose, who is a rebellious, 16-year-old African-American slave, and Ruth, a desolate, plantation-owning white woman deserted by her husband. The first half, in which the imprisoned Dessa recaps her history of rape, mutilation and murder to a voyeuristic writer, is full of confusing flashbacks. It is only in the second half that the show hits its stride as Dessa, joining a band of runaway slaves, forms a tensely interdependent relationship with the equally fugitive Ruth.

Ahrens and Flaherty, the songwriting team behind Ragtime, know how to write good numbers: one love song, In the Bend of My Arm, is especially seductive. Andrew Keates's production does a first-rate job of creating a sense of space, with figures etched against a sun-kissed landscape, on a tiny studio stage. Even if Cynthia Erivo could afford to show a bit more of the charm that is said to offset Dessa's menace, it is always a pleasure to watch this highly talented performer on stage. Cassidy Janson also conveys Ruth's hunger for liberation, Edward Baruwa has a massive stateliness as her black lover, and the musical director, Dean Austin, leads from the keyboard with decisive verve. Yet the final impression is of a musical that feels like a hectic precis of an epic novel, too incident-packed to lead us into a radical reappraisal of black-white relationships.